Charge pump

ABSTRACT

A charge pump ( 1 ) for generating a first output voltage (Vo 1 ) between a first output terminal (E) and a reference terminal (D), and a second output voltage (Vo 2 ) between a second output terminal (F) and the reference terminal (D). The charge pump ( 1 ) further comprises a first input terminal (C) for inputting a DC voltage (V), a first storage capacitor (Cr 1 ) coupled between the first output terminal (E) and the reference terminal (D) and a second storage capacitor (Cr 2 ) coupled between the second output terminal (F) and the reference terminal (D). The charge pump ( 1 ) further comprises a first terminal (A) and a second terminal (B) for coupling a pump capacitor (Cp) to a first triplet of switches (S 1 , S 2 , S 5 ) and to a second triplet of switches (S 3 , S 4 , S 6 ). The first triplet of switches (S 1 , S 2 , S 5 ) selectively couples the first terminal (A) to either the first input terminal (C) or to the reference terminal (D) or to the second output terminal (F). The second triplet of switches (S 3 , S 4 , S 6 ) selectively couples the second terminal (B) to either the first input terminal (C) or to the reference terminal (D) or to the first output terminal (E).

The invention relates to a charge pump for generating a first outputvoltage between a first output terminal and a reference terminal and asecond output voltage between a second output terminal and the referenceterminal, the charge pump further comprising a first input terminal forinputting a DC voltage, a first storage capacitor coupled between thefirst output terminal and the reference terminal and a second storagecapacitor coupled between the second output terminal and the referenceterminal.

Charge pumps are very much used in portable devices supplied from abattery type source. They usually provide at their output DC voltageswhose magnitude depends on an input DC voltage. Charge pumps comprisecapacitors and switches, a state ON or OFF of any switch beingcontrolled by binary signals generated by a counter/decoder driven by aperiodical clock signal.

U.S. Pat. No. 5,237,209 describes a charge pump that generates a bipolarvoltage output comprising 11 switches and 4 capacitors. Two of thecapacitors are used as storage devices for the output voltages. Theother two capacitors are periodically coupled to the input voltageterminals and output terminals via the switches. The order in which theswitches are ON or OFF is determined by a counter, the counter beingdriven by a periodical binary signal i.e. Clock. At the circuit input isapplied a DC voltage V. The circuit is capable to generate a bipolaroutput voltage that is substantially equal to double the input voltage Vi.e. it generates the output voltages +2V and −2V. It is observed thatthere are devices as displays that need to be supplied by anasymmetrical voltage as −V, +2V that still have a low cost and ifpossible occupying a reduced area within a chip.

It is therefore an object of present invention to provide a low costcharge pump generating balanced or unbalanced bipolar voltage.

In accordance with the invention this is achieved in a device asdescribed in the introductory paragraph, said charge pump furthercomprising:

-   -   a first terminal and a second terminal for coupling a pump        capacitor to a first triplet of switches and to a second triplet        of switches,    -   the first triplet of switches selectively coupling the first        terminal to either the first input terminal or to the reference        terminal or to the second output terminal,    -   the second triplet of switches selectively coupling the second        terminal to either the first input terminal or to the reference        terminal or to the first output terminal.

It is observed that the total necessary number of switches is 6 that issubstantially less then the switch number in the prior art. Furthermorethe charge pump comprises only 3 capacitors instead of 4. Two capacitorsare used as storage devices for the output voltages and one is used forgenerating the output voltages. Using less capacitors and switches thecharge pump according to the invention is cheaper and when integratedoccupies less area than the prior-art circuit.

In an embodiment of the invention a vector of signals is generatedperiodically with a period controlled by the reference periodicalsignal. A cycle for generating bipolar voltages either symmetric orasymmetric does not change from one design to another and therefore thevoltages are generated with the same efficiency. The switches could beof electromagnetic type, phototransistors controlled by a radiationsignal, bipolar transistors controlled by voltages or currents, CMOStransistors controlled by voltages. One would choose a switch type oranother depending on a specific application. For example, for portablehandsets it is desirable to have a small voltage generator and ifpossible integrated in the same chip with the supplied device. Thereforea possible solution could be use of CMOS transistors that could beeasily integrated.

In another embodiment of the invention a charge pump according to theinvention is used in a supplying system. The supplying system comprisesthe charge pump coupled to a LCD display, said display being suppliedvia the first output voltage and via the second output voltage. Thesupplying system is rather flexible because it could provide bipolarboth symmetric and asymmetric output voltages and could be easilyadapted to different type of displays because the output voltages couldbe easily provided to any of the outputs i.e. the first output voltagecould be either positive or negative having a voltage magnitude ofeither V or 2V.

The above and other features and advantages of the invention will beapparent from the following description of the exemplary embodiments ofthe invention with reference to the accompanying drawings, in which:

FIG. 1 depicts a detailed diagram of a charge pump according to theinvention.

FIG. 2 depicts a supplying system according to the invention.

FIG. 1 depicts a detailed diagram of a charge pump according to theinvention.

The charge pump 1 generates a first output voltage Vo1 between a firstoutput terminal E and a reference terminal D, and a second outputvoltage Vo2 between a second output terminal P and the referenceterminal D. The voltages have different signs i.e. one is negative andthe other is positive. The charge pump 1 further comprises a first inputterminal C for inputting a DC voltage V. The charge pump 1 includes afirst storage capacitor Cr1 coupled between the first output terminal Eand the reference terminal D and a second storage capacitor Cr2 coupledbetween the second output terminal F and the reference terminal D. Thecharge pump 1 also includes a first terminal A and a second terminal Bfor coupling a pump capacitor Cp to a first triplet of switches S1, S2,S5 and to a second triplet of switches S3, S4, S6. The first triplet ofswitches S1, S2, S5 selectively couples the first terminal A to eitherthe first input terminal C or to the reference terminal D or to thesecond output terminal F. The second triplet of switches S3, S4, S6selectively couples the second terminal B to either the first inputterminal C or to the reference terminal D or to the first outputterminal E. A phase generator/decoder 10 generates a vector of signals P(Phase) synchronized with a reference periodical signal Ck said vectorof signals controlling a state of the switches i.e. ON or OFF. Thecharge pump 1 generates one of the combination (−V, 2V), (−2V, 2V), (−V,V), (V, −V), (2V, −2V), (2V, −V) for the first output voltage Vo1 andthe second output voltage Vo2 respectively. It is observed that thecharge pump 1 uses only 3 capacitors Cp, Cr1 and Cr2 i.e. one less thanin the cited prior art. Furthermore the total number of switches is 6i.e. less than in the cited prior art. Therefore, the charge pump 1 ofthe present application offers a cheaper solution than that used in thecited prior art. The phase generator/decoder 10 generates a vector ofsignals having four components i.e. Phase 1, Phase 2, Phase 3 and Phase4.

In Table 1, the switching sequence of the switches is presented.

TABLE 1 Vo2 Vo1 Phase 1 Phase 2 Phase 3 Phase 4   −V +2 V S2, S3 S1, S4S2, S3 S5, S6 −2 V +2 V S2, S3 S1, S4 S2, S4 S5, S6   −V   +V S2, S3, S4S5, S6 S2, S3, S4 S5, S6   +V,   −V S1, S5, S6 S2, S4 S1, S5, S6 S2, S4+2 V −2 V S1, S6 S3, S5 S5, S6 S2, S4 +2 V   −V S1, S6 S3, S5 S1, S6 S2,S4

In Table 1 in any line are indicated the voltage values of the firstoutput voltage Vo1 and the second output voltage Vo2. In any column ofTable 1 there are indicated which of the switches are in an ON state,all the other switches being in an OFF state. Let us consider that wewant to obtain Vo1=+2V and Vo2=−V. It results from Table 1 that thefirst component of the vector of signals Phase 1 command switches S2 andS3 in an ON state. It results that the pump capacitor Cp is charged at avoltage V having a plus at the terminal B and a minus at the terminal A.When the second component of the vector of signals Phase 2 is appliedthe switches S1 and S4 go in an ON state. Therefore first output voltageVo1 becomes +2V, the voltage being stored in the first storage capacitorCr1. The signal Phase 3 determines the switches S2 and S3 to go in an ONstate, the pump capacitor being charged is the same way as with thesignal Phase 1. Finally, when the signal Phase 4 is applied the switchesS5 and S6 go in an ON state and the voltage across the second storagecapacitor Cr2 is −V. The process in continued with another cycle Phase 1. . . Phase 4 and so on. It is observed that at one moment only onecomponent of the vector of signals Phase is active i.e. in a state thatdetermines an ON state of a switch. The control signals could begenerated using an arrangement comprising a binary counter coupled to adecoder 1 of 4 the arrangement being known per se.

Any switch has an input terminal, an output terminal and a controlterminal. Any switch could be either in an ON state or in an OFF state.In an ON state any switch connects the output terminal to the inputterminal. In an OFF state there is no connection between the outputterminal and the input terminal. The state of any switch is determinedby a control signal applied to the control terminal. Any switch could beof electromagnetic type, phototransistors controlled by a radiation,bipolar transistors controlled by voltages or currents, CMOS transistorscontrolled by voltages. Depending on the switch type, the control signaldetermining an ON state could be either logical 1 or logical 0 andrespectively logical 0 or logical 1 determining an OFF state of theswitch. One would choose a switch type or another depending on aspecific application. For example, for portable handsets it is desirableto have a small voltage generator and if possible integrated in the samechip with the supplied device. Therefore a possible solution could beuse of CMOS transistors that are relatively easy to be integrated.

It is further observed that the vector of signals Phase is generatedperiodically with a period controlled by the reference periodical signalCk. This situation is characteristic for a burst mode of operation ofthe charge pump 1. In a controlled mode using known techniques severalparameters as stop switching, switching frequency and duty cyclechanging could be controlled. In a stop switching situation, when anoutput voltage reaches a predefined value the switches charging thatoutput are no longer switched ON until the output voltage becomes lowerthan the predefined voltage. The charge transfer between the pumpcapacitor Cp and the storage capacitors Cr1, Cr2 depends on thefrequency and duration of the vector of signals Phase and thereforedepends on the Clock. Hence, controlling the clock of phasegenerator/decoder 10 the charge transfer could be controlled. It couldbe defined a duty cycle between charging the pump capacitor Cp i.e.Phase 1 and Phase 3 and discharging the pump capacitor Cp i.e. Phase 2and Phase 4. This parameter also controls the charge transfer betweenthe pump capacitor Cp and the first storage capacitor Cr1 and the secondstorage capacitor Cr2.

Charge pumps generating bipolar voltages are also disclosed in e.g. U.S.Pat. Nos. 5,262,934 and 5,461,557. These charge pumps use 4 capacitors,two pump capacitors and two storage capacitors and more switches than inthe present invention. Therefore they have a higher price and use morechip area than the charge pump 1 according to the present invention.

It should be pointed out that the Table 1 could be used for generatingthe vector of signals Phase for the combination of output voltagesindicated there. This could be realized in a device comprising forinstance a counter coupled to a memory, said memory storing arepresentation of Table 1. It is further observed that the signalsgeneration according to the Table 1 could be implemented in a softwareprogram. The program could run on a controller that generates the vectorof signals Phase according to Table 1. It should be emphasized that theoutput voltages could be either positive or negative and the charge pump1 generates bipolar voltages either equal to each other or not. Hencethe charge pump 1 could be used in both applications requesting asymmetrical bipolar voltage supply or in applications requestingasymmetric voltage supply.

Such a situation is depicted in FIG. 2. The charge pump 1 is coupled toa battery 100, the battery 100 generating the voltage V. The charge pump1 generates the output voltages Vo1 and Vo2 for supplying a LCD display200, said LCD display 200 needing an asymmetric bipolar supplyingvoltage i.e. +2V and −1V. It is observed that if a load of the chargepump 1 needs supplying symmetrical bipolar voltages that could be foundin Table 1 these voltages could be generated using the charge pumpaccording to the present invention.

It is observed that depending on the logic circuit used to control thetriplets of switches the output voltages could be different than thatshown in Table 1, but a power efficiency of the charge pump would belower. Additional voltages, other than that claimed in claim 3 could beobtained using more pump capacitors and switches in a straightforwardmanner. Furthermore, depending on a load, the charging sequencedescribed in Table 1 could be modified for maintaining a relativelyconstant voltage at any output of the charge pump.

It is remarked that the scope of protection of the invention is notrestricted to the embodiments described herein. Neither is the scope ofprotection of the invention restricted by the reference numerals in theclaims. The word ‘comprising’ does not exclude other parts than thosementioned in the claims. The word ‘a(n)’ preceding an element does notexclude a plurality of those elements. Means forming part of theinvention may both be implemented in the form of dedicated hardware orin the form of a programmed purpose processor. The invention resides ineach new feature or combination of features.

1. A charge pump for generating a first output voltage between a firstoutput terminal and a reference terminal and a second output voltagebetween a second output terminal and the reference terminal, comprising:a first input terminal adapted to provide a direct current (DC) voltage;a first storage capacitor coupled between the first output terminal andthe reference terminal and a second storage capacitor coupled betweenthe second output terminal and the reference terminal; and a firstterminal and a second terminal adapted to couple a pump capacitor to afirst triplet of switches and to a second triplet of switches, wherein:each of the switches of the first triplet of switches selectivelyconnects the first terminal directly to a respective one of the firstinput terminal, the reference terminal or to the second output terminal,and the second triplet of switches selectively couples the secondterminal to either the first input terminal or to the reference terminalor to the first output terminal.
 2. A charge pump as claimed in claim 1,wherein each of the switches of the second triplet of switchesselectively connects the second terminal directly to a respective one ofthe first input terminal, or the reference terminal or the first outputterminal.
 3. A charge pump as claimed in claim 1, further comprising aphase generator/decoder for generating a vector of signals synchronizedwith a reference periodical signal said vector of signals controlling astate of the switches.
 4. A supplying system adapted for use in a liquidcrystal display (LCD), comprising: a charge pump adapted to generate afirst output voltage between a first output terminal and a referenceterminal and a second output voltage between a second output terminaland the reference terminal, wherein the charge pump further comprises: afirst input terminal adapted to provide a direct current (DC) voltage; afirst storage capacitor coupled between the first output terminal andthe reference terminal and a second storage capacitor coupled betweenthe second output terminal and the reference terminal; and a firstterminal and a second terminal adapted to couple a pump capacitor to afirst triplet of switches and to a second triplet of switches, wherein:each of the switches of the first triplet of switches selectivelyconnects the first terminal directly to a respective one of the firstinput terminal, the reference terminal or to the second output terminal,and the second triplet of switches selectively couples the secondterminal to either the first input terminal or to the reference terminalor to the first output terminal.
 5. A supplying system as claimed inclaim 4, wherein the first output voltage and the second output voltagesupply the LCD.